May I recommend MedMod? The 2.04 version (Abbey road) does not change any units, but it improves the AI dramatically. The problem with the AI is that it does not build economic improvements. That's why it tends to be dirt poor. And the AI is programmed to build peasants when it doesn't have any money. WesW has reprogrammed the AI and made several changes to the game with the AI in mind to create a more powerful opposition. This resulted in a AI which builds more economic improvements, trades more, thus has more money and builds more powerful armies.

You can find WesW's MedMods on this site Wes Whitaker's Total Modification Site.
V2.04 it the version that improves the AI while not changing units. It creates four new campaign next to the existing ones, so you do not need a clean install of MTW. The mod does require VI.

MedMod v3.14 (Total Immersion) is a total modification, it does change units and countries. But I understand WesW is working on MedMod v4, a modification that will be even better. Wes himself described as 'the game MTW ought to have been'. I am interested how that will turn out.